Refraction of Light

[Dita]

A cylindrical material of radius R = 2.00 m has a mirrored surface on its right half, (as in figure that i have attached below). A light ray traveling in air is incident on the left side of the cylinder. If the incident light ray and exiting light ray are parallel and d = 2.00 m, determine the index of refraction of the material.

I know that the index of refraction of a material n is n = c /u where c is the speed of light in a vacuum and u is the speed of light in the material.
But in the figure there is nothing which has to do with this formula.

A little help?

[Doc Al]

You'll need to apply Snell's law of refraction and a little geometry. Snell's law is:
n_1sin\theta_1 = n_2sin\theta_2